Warn if sent_box was not set

[josch]

I just lost a couple of emails I sent because I forgot to set sent_box in my alot config. Here a couple of solutions:

• Add a note to the docs saying that if sent_box == None (the default) then all sent messages will be discarded
• If an email is sent and sent_box == None, display a warning to the user asking him if he really wants to send and discard his email instead of first setting the sent_box configuration value to something unequal None. To this end, add an additional configuration option which suppresses this warning.
• Instead of defaulting sent_box to None, choose a reasonable default location for sent email. Maybe in ~/Sent or in ~/.config/alot/Sent or at least in /tmp? Combined with the warning message, the user can then retrieve sent email, add it to the proper location and set the sent_box option. There could be a configuration option default_sent_box which sent_box defaults to when equal to None?

I would supply a patch for the second option but there is this issue with the hooks being unable to stop a sending operation when depending on user input (#395).

Similar reasoning can be applied to the draft_box option.

[pazz]

i like the last solution best. the warning could be shown only if the sending Account's sent_box parameter is None
and tell the user to change that in order to get rid of this msg..

[pazz]

sorry, that’s your second proposition :)
Note that #395 has been resolved in the meantime. I think the best way to deal with this would be
to first send the mail and try to store. In case Account.store_mail returns None
(dealt with somewhere here: https://github.com/pazz/alot/blob/master/alot/commands/envelope.py#L241 ;
maybe it'd be nicer to let this raise a StoreMailError instead that is caught here?)
we could:

• inform the user that there is no maildir set and the copy will hence not be stored nor indexed locally
  and this can be remedied by setting the respective config option.
• ask whether to save a copy on disk (per ui.choice) and if so, ui.prompt a path, defaulting to $timestamp.eml
• write the email as is to that path unless prompt got aborted